Add Endangered filter to heritage search form

[zigzagdev]

Summary

Surface a filter for is_endangered so users can scope search results to endangered heritage sites.

Scope

• Domain
  • Add is_endangered: boolean | null to HeritageSearchParams in client/src/domain/types.ts (or boolean defaulting to false; nullability decided alongside parent)
  • Update parseHeritageSearchParams / serializeHeritageSearchParams in client/src/app/features/search/mapper/search-heritages.params.ts
• API
  • Wire the new param into client/src/app/features/search/apis/search-api.ts once backend support is confirmed
• UI
  • Add a single checkbox "Endangered only / 危機遺産のみ表示" to HeritageSearchForm. Off = no filter, on = endangered only.
  • Add endangeredOnly ui-text key to en / ja ui.json
• Tests
  • Param parse / serialize round-trip
  • Form interaction smoke test

Acceptance criteria

• Toggling the checkbox narrows the result list
• URL reflects the flag (e.g. ?is_endangered=true); empty / off state omits the param
• Tests cover the new field